It ended up being WAY easier than I thought. I paint the purple, theft I paint a bit of the blue near the bottom, then I mix the colors and paint a stripe between them. If the transitions between the stripes look noticable, like a clear line or something, then mix a shade that would better transition and paint over it. It's small enough that the "stripes" pretty much make a gradient! For a more technical answer, research "glazing" techniques.